98 POTASH. 



perhaps the most important of all their compounds, 

 a very great number of them are bases (109). 



198. The class of substances called bases, or those 

 with which acids can combine, is very numerous. 

 The commonest, and therefore the most important, 

 are the three alkalies — potash, soda, and ammonia, 

 together with a few substances called earths, because 

 they are the principal ingredients which enter into 

 the composition of the earth or soil. The nature and 

 properties of ammonia have already been considered 

 (148). 



199. Potash never occurs pure in a free state; its 

 most abundant source is the vegetable kingdom, and 

 the simplest method of procuring it is by burning 

 wood. When this is done, after all the combustible 

 matter is burned, there remains a quantity of a white 

 or gray substance called ashes, which when put into 

 water forms a caustic solution. This is caused by the 

 presence of potash, which, not being altered by the 

 heat occasioned by the burning of the wood, is left 

 in the ashes. 



200. In this way, however, we do not obtain the 

 potash pure, because, being a powerful base, and 

 having a strong affinity for acids, it combines with 

 some of the carbonic acid formed by the burning of 

 the carbon contained in the wood, and therefore the 

 caustic substance found in the ashes of the wood is 

 an impure carbonate of potash. It is obtained tole- 

 rably pure by pouring water on wood ashes, straining 

 oflf the clear liquor, and evaporating it ; a white salt 



